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6
3 Hardware Configuration
3.1 Pull-up Resistors
I
2
C Bus systems are based on open-collector technology requiring pull-up devices
on each signal wire. These pull-up devices usually take the form of pull-up resistors
connected to bus power.
The I
2
C adapter includes a slide switch used to enable or disable internal +5VDC
I
2
C Bus 1.8K ohm pull-up resistors attached to the SCL, SDA, and /INT lines.
Every I
2
C Bus system must have at least one pull-up on the signal lines. Use this
switch to configure the pull-up resistors for your system.
3.2 Connecting to a 3.3 Volt Target System
If you are connecting the I
2
C adapter to a 3.3 volt target system, you should follow
these steps BEFORE applying power:
• Shut off the I
2
C adapter’s internal pull-ups (See Pull-up Resistor section). Use
external pull-ups to the target system’s 3.3V power. These pull-ups may already
be present in the target system.
• Disconnect the I
2
C connector +5V wire from the target system. The I
2
C adapter
will be powered from its own +5V power supply, and the target system will be
powered by its own 3.3V power supply.
The I
2
C adapter is a 5-volt device. Any signal above 3.3V on the SCL, SDA, and
/INT lines is high enough for the adapter to see a Logical 1.
3.3 Connecting to an SMBus Target System
If you are connecting the I
2
C adapter to a SMBus target system, you should follow
these steps BEFORE applying power:
• Shut off the I
2
C adapter’s internal pull-ups (See Pull-up Resistor section).
• Use external SMBus rated (appoximately15k ohms) pull-up resistors. These
pull-ups may already be present in the target system.
• Visit our I
2
C versus SMBus FAQ page (www.mcc-us.com/I2CSMBusFAQ.htm).
• See the SMBus Specification for additional details.
Special Note for SMBus Users: MCC’s I
2
C adapters are designed to be I
2
C Bus